1. Field of the Invention
The invention relates to the field of liquid level displays. It proceeds from a capacitive liquid level sensor according to the preamble of claim 1.
2. State of the Art
A multiplicity of devices which are based on very different physical measuring principles are known in the prior art for determining the liquid level of a container. These principles comprise electric (capacitive or resistive) and optical methods, radar reflection methods, ultrasound echo-time methods and gamma absorption methods.
In offshore crude oil recovery, use is made of so-called separation tanks in which the various phases (sand, water, oil and gas) occurring during drilling or conveyance are separated on the basis of their differences in density and led off into separate pipe systems. It is very important in this case to know the height of the separating layer between the water and oil, in order to be able to open and close the drain valves of the two media on the tank in a controlled fashion. This requires reliable liquid level measuring instruments. If such a liquid level measuring instrument does not function, or functions incorrectly, it is possible, for example, for oil to pass into the water outlet and cause severe environmental damage and high costs.
Recently, high-pressure separation tanks have been developed which are suitable for operation on the sea floor a few 100 m below the surface of the sea. The conveyed and already separated oil can then be pumped to the surface of the sea with a much lower expenditure of energy. Such separator tanks are exposed to very high pressures of 60-180 bar, specifically, from outside, to the water pressure at the seabed and, from inside, to the pressure of the conveyed crude oil, and to high temperatures of 50-120.degree. C. The liquid level measuring system must function for years without maintenance and reliably under these difficult conditions, since an operational failure and premature replacement would entail high costs.
The prior art discloses multifarious electrode arrangements for capacitive measurement of the electrode environment, and thus for determining the liquid level. For example, rod-shaped measuring probes are proposed which have a plurality of electrodes arranged along a probe axis and make use, in particular, of the large discontinuity in the dielectric constant at the boundary layer between oil and water. A disadvantage of the known measuring probes is that the spatial resolution is limited by the periodicity of the electrode arrangement, and that liquid levels situated between cannot be measured. Again, it is not possible to detect boundary layers between media having the same dielectric constants. For media of different conductivity, it is known to measure the ohmic discharge currents between individual electrodes in order to locate the boundary layer. This requires a measuring frequency to be selected to be so low that the ohmic current dominates the capacitive one in both media. However, it has not so far been taken into account that the conductivity of the media can greatly influence the field of distribution between the electrodes, and thus the magnitude of the capacitances.
Soiling of the measuring probe by the media to be measured constitutes a serious problem for the measuring accuracy and reliability of such liquid level sensors. For example, a conductive water film can interfere with or render impossible the detection of an oil medium, and a non-conducting oil coating can do the same for the detection of a water medium.
DE 28 19 731 discloses a capacitive sensor which serves to monitor the limiting value of an adhesive, conductive medium. The capacitance is measured between a rod-shaped probe and the container wall. It is possible for conductive deposits on the probe and the wall to cause fault currents which are picked up by a shield electrode along the probe axis. The fault currents can also be kept small by means of measuring frequencies which are as high as possible. A disadvantage of this arrangement consists in that the liquid level height cannot be measured continuously.
DE 38 12 687 A1 presents a capacitive sensor for simultaneously determining the liquid level and the water content of a liquid which is at least weakly conductive. For this purpose, two coaxially arranged electrodes are dipped into the medium and the complex impedance is measured at at least two frequencies. The capacitive reactance is a measure of the liquid level, and the ohmic effective resistance is a measure of the conductivity of the medium.
None of the above-named sensors takes account of the fact that the measurement of capacitance in conductive media is influenced by ionic charge transport, from which new possibilities arise for optimizing the electrode geometry. Furthermore, it has not so far been possible to eliminate the disturbing influence of insulating dirt coatings on the measuring probe.